1. Field of the Invention
This invention relates to an improved process for the polymerization of mixtures of cis and trans-isomers of 1,3-pentadiene in the production of block copolymers. More specifically this process permits better utilization of the cis-isomer of 1,3-pentadiene, thus increasing the amount of total pentadiene polymerized, by continuously or incrementally adding a polymerization promoter consisting essentially of a vinyl-substituted aromatic monomer using a hydrocarbon solvent having no added aprotic solvents of a polar nature, such as tetrahydrofuran, and not exceeding a polymerization temperature of about 60 degrees C. in the absence of added polymerization promoter or about 70 degrees C. in the presence of added polymerization promoter.
2. Description of The Prior Art
The prior art contains many references to block copolymers of vinyl aromatics and conjugated dienes and processes for preparing them. Butadiene and isoprene are the most commonly taught conjugated dienes. 1,3-pentadiene (piperylene) is also often included in the same grouping as an acceptable conjugated diene monomer. Poly(1,3-pentadiene) is known to be more ozone resistant than other diene polymers, for example, polybutadiene or polyisoprene.
U.S. Pat. No. 3,030,346 teaches initial charging of both conjugated diene and vinyl-substitued aromatic compound in the presence of a hydrocarbon diluent and organolithium compound and effecting the polymerization at two temperature levels, to 100 degrees Fahrenheit (F.), initially to copolymerize the conjugated diene and part of the vinyl-substituted aromatic, then 115 degrees F. to 140 degrees F. to polymerize the rest of the vinyl-substitued aromatic compound. The period of time at the first temperature must be sufficient to polymerize at least 85 percent of the conjugated diene or a minimum time period of 6 hours.
1,3-pentadiene has both cis and trans-isomers with the trans-isomer being the more reactive of the two isomers. 1,3-pentadiene refined from petroleum crude contains about 2/3 trans-isomer and about 1/3 cis-isomer. With only a 2 degree centigrade (C.) difference in boiling points (cis-isomer 44 degrees C., trans-isomer 42 degrees C.) the separation of these two isomers is difficult and commerically uneconomical.
R. P. Shatalov, L. V. Kovtunenko and N. I. Simirnov (ISSLEDOVANIA V OBLASTI FIZIKI I KHIMII KAUCHUKON I REZIN. LENINGRAD. V. (3)25-9(1973)) studied various copolymer based on piperylene using piperylene containing 99 weight percent trans-isomer. Since the trans-isomer is significantly more reactive than the cis-isomer, Shatalov et al.'s use of only the trans-isomer is not unexpected. If a mixture of isomers had been used, an inordinate amount of time would be required to complete polymerization of the mixed isomer mixture.
Traditionally, to those skilled in the art of anionic block copolymerization, all monomers are expected to be totally consumed during the polymerization. Use of a mixed isomer mixture would require an extreme amount of time to totally consume the cis-isomer.
Increasing the temperature to increase the rate of cis-isomer polymerization results in premature chain termination. This premature chain termination means the desired block structure will not be obtained in acceptable yields.
The addition of polar additives, such as tetrahydrofuran, other ethers or tertiary amines, also results in premature chain termination, and undesirable polymer microstructure and does not promote the overall 1,3-pentadiene polymerization rate. While styrene does copolymerize with both isomers of 1,3-pentadiene, the fact that a vinyl-substituted aromatic monomer copolymerizes with the cis-isomer allows use of a mixed isomer mixture of 1,3-pentadiene for polymerization.
Large residues of 1,3-pentadiene would interfere with the polymerization of a glassy styrenic end block. Use of a polymerization promoter, such as styrene, during the formation of an elastomeric block containing 1,3-pentadiene allows the use of a monofunctional organolithium initiator to produce an ABC triblock copolymer, where A and C are non-identical glassy end blocks and B is an elastomeric center block, and the use of a difunctional or multifunctional organolithium initiator to polymerize the elastomeric center block then adding a monomer or monomers and polymerizing the end blocks. Also, by controlling the rate and time of addition of the polymerization promoter, tapered block copolymers, initially rich in 1,3-pentadiene, then becoming increasingly rich in styrene, can be produced.
It is an object of this invention to provide a method of making triblock copolymers, using organolithium initiators, having an elastomeric center block polymerized from a mixed isomer mixture of 1,3--pentadienes using a cis-isomer 1,3-pentadiene polymerization promoter.
It is another object of this invention to provide a method of making tapered block copolymers from vinyl-substituted aromatic compounds and a mixed isomer mixture of 1,3-pentadiene utilizing a cis isomer 1,3-pentadiene polymerization promoter.
It is still another object of this invention to provide block copolymers containing 1,3-pentadiene that have improved heat and exposure stability.